High-resolution Interferometric Diagnostics for Ultrashort Pulses

4. EXPERIMENTAL IMPLEMENTATIONS OF MULTIPLE SPECTRAL SHEARINGINTERFEROMETRYsurements and the difference in their group-delay dispersion equalled the known value for theLASF31A glass. Finally the RMS time-bandwidth product (defined using angular frequency ω)of the reconstructed pulse was 11.9, one of the largest ever measured using a spectral shearingmethod.Having experimentally proven the principle of multiple-spectral shearing interferometry, Inow describe a means of acquiring multiple shears simultaneously.4.2 Simultaneous acquisition of shearsSequential acquisition prohibits rapid updates and requires the shears to be chosen in advance.This motivates the simultaneous acquisition of a range of shears in a two-dimensional data set.One potential way of acquiring multiple shears simultaneously is by adding more arms to theinterferometer in a SPIDER. This would incur extra complexity and would require some formof multiplexing for each shear to be detected. Another approach is a chirped-arrangement forSPIDER (CAR-SPIDER), introduced in section 2.3.6.2. Here, I used a spatially encoded arrangementfor CAR-SPIDER, a new implementation developed during the course of this dissertation[282].The work presented in this section formed part of the article “Resolution of the relative phaseambiguity in spectral shearing interferometry of ultrashort pulses” [267].4.2.1 Spatially encoded arrangement chirped-arrangement SPIDERThe operating principle of the SEA-CAR-SPIDER is depicted in Fig. 4.6. The test pulse (TP) E (ω),propagating along the z -axis, is sum-frequency mixed with two ancilla beams A & B which arespatially chirped along the x-axis and crossing at an angle θ in the xz-plane. The spatial chirpsare oppositely aligned, such that the local frequencies of the two ancillae are ω A (x )=ω up + αxand ω B (x )=ω up − αx where ω up is their common frequency at x = 0 and α the degree of spatialchirp. Two noncollinear replicas of the test pulse are produced, upconverted by the local ancillafrequency. These are reimaged onto the entrance slit of an imaging spectrometer, producing spa-98